Calipers

A vernier caliper is a handheld measuring tool designed for measuring distances, allowing for the measurement of external and internal distances and depths. The most common accuracy of vernier calipers is 0.1-0.02 millimeters. Typically, vernier calipers are used to measure perpendicular distances, external and internal diameters of objects, depths of holes, or lengths of shoulders. Vernier calipers are commonly used across various fields, such as the metal industry, woodworking, science, and medicine.

When using an analog vernier caliper equipped with a vernier scale, the object to be measured is clamped between the caliper's measuring jaws, and the measurement result is read from the main scale, which indicates whole millimeters, and a separate vernier scale, which shows decimals of a millimeter (0.1mm). Also available on the market are dial calipers, in which the measurement reading is obtained from the position indicated by the dial hand.

Modern digital calipers are easy to read, providing measurement results with an accuracy of one-hundredth of a millimeter (0.01mm). However, when using a caliper, it's important to consider the instrument's set accuracy and the user's measurement uncertainty, such as the measurement force, as even a digital caliper cannot guarantee reliable results with 0.01mm accuracy. In machine shops, digital calipers are often IP67 rated, which means they are completely protected from dust and can withstand immersion in water for 30 minutes at a depth ranging from 15 centimeters to 1 meter. Thanks to the IP67 rating, vernier calipers can also be used in environments where exposure to cutting fluid from a machine tool cannot be avoided. Modern digital calipers often feature wireless data transfer, enabling easy creation of measurement reports, such as those in an Excel-based measurement log.

Calipers are the most common measuring tool in machine shops and are typically part of a machinist's personal toolset. The most common calipers have a measurement range of less than 200mm, but calipers with a measuring range of several meters are also available, known as workshop calipers. There are also several versions of calipers available, such as depth calipers, thread calipers, groove calipers, and they can be equipped with various measuring jaw options for different applications, such as measuring pipe walls, measuring diameters of internal grooves, etc.

The condition of the caliper's measuring jaws and tips can be checked by closing the jaws and holding the caliper against the light to see if any light passes through the measuring surfaces of the jaws and tips. If no light is visible, the caliper is in good condition regarding the measuring jaws and tips. When using a vernier scale-equipped caliper, it is also essential to check that the scale reads zero when the jaws are closed. The condition of the depth gauge at the end of the caliper is checked by ensuring it is level with the body of the caliper when the jaws are closed.

The maintenance of the sliding and measuring surfaces of a caliper should always be carried out before use by wiping off dust and dirt with a soft, dry cloth. After use, the caliper should be cleaned of oil and water and oiled. The caliper should not be stored in direct sunlight, at too high or too low temperatures, or in a humid environment. Also, the caliper jaws should not be completely closed during storage.

The caliper should be handled carefully to avoid damaging its sharp measuring tips. Identification numbers or other markings should not be engraved on the caliper using marking devices.

Parts of a Caliper

The parts of an analog caliper equipped with a vernier scale are:

1. Frame
2. Slider
3. Fixed jaw
4. Movable jaw
5. Internal measuring tips
6. Depth gauge
7. Main scale
8. Vernier scale
9. Locking latch

In addition, the caliper may have different measuring tips, a scale graduated in inches, or a locking screw. Except for the scales, a digital caliper contains the same parts, but the scales are replaced with a digital display.

Reading a Caliper

Reading an analog caliper is done using the main scale and the vernier scale. First, read the full millimeters indicated by the zero line of the vernier scale on the main scale (1). Then, read the fractional millimeters from the vernier scale at the point where a vernier scale division aligns with a main scale division (2). The measurement result is obtained by adding the full and fractional millimeters. Full millimeters 24 + fractional millimeters 0.4 = 24.4mm

When measuring with a caliper, various errors can occur, such as excessive measuring force, thermal expansion due to temperature differences between the caliper and the workpiece, the parallax effect of calipers equipped with a vernier scale, and the influence of the thickness of the internal measuring jaws on measuring small hole diameters. Errors may also occur when measuring an object at the base or tips of the jaws.

When using manually-equipped vernier scale calipers, it's important to ensure the caliper is perpendicular to the measurer. If the vernier scale is read from an angle, reading errors may result due to the parallax effect caused by the height difference between the main scale and the vernier scale.

In internal measurements, the internal measuring tips should be positioned as deep as possible, and the largest reading observed during the measurement is recorded. When measuring depth, the smallest reading obtained during the measurement is recorded.

Errors may also occur if the movable jaw tilts compared to the fixed jaw due to straightness errors of the caliper frame or excessive measurement force. Calipers are typically made of stainless steel, so it's important to note that when measuring materials other than steel, the thermal expansion coefficient of the caliper is different from that of the material being measured.